Conventionally, as a so-called run-flat tire which can travel a certain amount of distance safely without losing a load supporting ability of the tire even in the state in which the inner pressure of the tire is decreased due to a puncture or the like, a variety of run-flat tires of a side reinforced type have been proposed wherein a crescent cross-sectional side reinforcing rubber layer having a relatively high modulus is disposed in the inner side of a carcass in the sidewall portion, thereby improving the rigidity of the sidewall portion, and allowing the tire to support loads without extremely increasing the flexural deformation of the sidewall portion when the inner pressure is decreased.
However, in a conventional side reinforcing type run-flat tire, since deformation of the tire is large during run-flat running and the temperature of a sidewall portion becomes high, and the stiffness of the sidewall portion decreases due to softening of the rubber, causing a further increase in the deformation, the main reason for failure at the end of run-flat running has been due to a crack in the above-mentioned crescent cross-sectional side reinforcing rubber layer. For this reason, the conventional side reinforcing type run-flat tire has a problem in that the durable distance during run-flat running is short.
On the other hand, when the sidewall portion is reinforced by making the gauge of the side reinforcing rubber layer thick or the like in order to prolong the durable distance of the tire during run-flat running, there has been a problem in that the tire weight increases, the vertical stiffness of the tire during normal running increases, thereby deteriorating the ride quality during normal running.
As technologies for attaining the combination of the run-flat durability and ride quality for a run-flat tire, for example, in Patent Document 1, disclosed is a run-flat tire in which the inclination angle of a cord with respect to the tire radius direction is 0 to 20°, and a reinforcing cord layer having a stiffness equivalent to or morethan that of a carcass is provided such that the layer is at least adjacent to a carcass portion from the belt end to the maximum width portion of the tire side portion and such that the layer covers the reinforcing rubber layer from the outer side of the carcass. In Patent Documents 2 and 3, technologies are disclosed in which, in a side reinforcing type run-flat tire, a polyketone fiber cord which is composed of a plurality of twisted filament bundles formed of polyketone and in which the thermal shrinkage stress σ at a temperature of 177° C. and the elastic modulus E at a temperature of 25° C. under the conditions of a load of 49N satisfy prescribed conditions is used for a cord reinforcing layer which is arrangedon at least a part of a region A from the belt end to the maximum width portion and a region B from the vicinity of a bead core to a bead filler.